Hydrothermal Synthesis, Structure, and Magnetic Properties of a Layered Fe(III) Carboxymethylphosphonate: [Fe(H2O)(O3P-CH2-CO2)] or MIL-49

Fe^III(H₂O)(O₃P- (CH₂)-CO₂)] or MIL-49 was hydrothermally synthesized under autogenous pressure at 170°C for 48 h. Its bidimensional structure was solved from single-crystal X-ray diffraction in the monoclinic space group P2₁/c (No.14). Cell parameters are a =9.3836(3) Å, b=6.3798(3) Å, c=10.1371(3) Å, β=111.891(2)°, Z=4, and V=563.10(4) ų. Reliability factors of the structure refinement are R₁(F)=0.0470 and wR₂(F²)=0.1297 for 1036 reflections with I>2σ(I). The structure of MIL-49 is based on inorganic units built up from two edge-sharing FeO₅(H₂O)] octahedra and two carboxymethylphosphonate groups. The connection of these units leads to the formation of hybrid sheets. A dangling oxygen atom from the carboxy function points toward the interlayer space and is responsible for hydrogen bonding with adjacent layers. Magnetic measurements and ⁵⁷Fe Mössbauer study reveal an antiferromagnetic ordering below T_N=25(1) K which becomes canted below 11(1) K.